1. Field of the Invention
The present invention relates to a femoral component for a hip prosthesis to be used in total hip arthroplasty. More specifically, it relates to a femoral component having a shape which provides a better fit with the femoral medullary canal.
2. Description of the Prior Art
It is possible to eliminate severe pain in hip joints as a result of arthritis or other infirmities by implanting a stem within a femoral intramedullary canal. A ball on the proximal end of the stem cooperates with a prosthetic socket implanted in a prepared natural acetabulum and thereby provides for articulation between the femur and the acetabulum after the femoral head has been resected and the proximal femur cleared of cancellous bone proximally and fatty tissue distally.
In order to maintain pain-free articulation of the hip joint following prosthesis implantation, it is essential that fixation be attained between the stem and healthy bone bed. Such a fastening is accomplished by several methods. A polymethylmethacrylate cement can act as a bone between the implant and the interstices of the prepared bone bed. A second method known as press fit relies on the dimensional difference between the implant and prepared bone bed for stability. A third fixation method provides a 3-dimensional texture on the implant's surface that, when used in a "line to line" or slightly undersized prepared bone bed, allows for bony ingrowth texture.
Various patents relate to a femoral component for press fit with and biological fixation to the wall of the proximal metaphysis and intramedullary canal. For example, U.S. Pat. No. 4,589,883 teaches a proximal portion which is elliptical in cross-section with its major and minor axes twisted along the proximal direction. U.S. Pat. No. 4,435,854 (now Reissue Patent 32,471) relates to a hip joint prosthesis having a stem which, in its proximal region, has a curvature in the anterior-posterior plane with a center of curvature anteriorly and, in its distal region, includes a shank having a curvature in the opposite direction in the anterior-posterior plane, i.e. at its center of curvature posteriorly. This design supposedly reduces the tendency of the shank to turn within the medullary canal.
U.S. Pat. No. 5,002,580 relates to a femoral component designed to provide a non-uniform interference press fit with the intramedullary canal. With this design, the femoral canal is prepared in such a manner that the prosthesis has line-to-line contact with cortical bone on the lateral side and produces an interference fit with the softer cancellous bone on the medial side of the femur. U.S. Pat. No. 4,813,963 relates to a femoral component for a hip prosthesis including a proximal portion with an asymmetric contour defining an anterior side which forms an acute angle with a lateral side and the posterior side approaches the anterior side in a direction of the medial side. Furthermore, the medial side is arcuate in shape while the other sides include linear edges in cross-section.
While the prior art femoral components have provided a stem and surgical techniques which produced acceptable results, improved stem fit and a simplified surgical procedure are still desirable.
In addition to varying proximal geometries, a femoral prosthesis can be broadly characterized as also having straight or curved stems. Straight prostheses are left/right interchangeable and are less technically demanding to implant than curved stem prostheses. On the other hand, curved stem prostheses, while being somewhat more demanding for the surgeon to implant, provide the advantage of an improved fit within the prepared femoral canal. The design of the present invention possesses a unique geometry which provides the superior proximal fit afforded by a curved stem prosthesis, but can utilize the proven canal preparation techniques employed in the implantation of straight stem prostheses. The prostheses of the present invention can be broken into three regions and can be described geometrically as a distal parabolic section, a conical mid-stem section and a proximal geometry which medially and anteriorly conforms to the anatomy of a natural femoral medullary canal, and posteriorly and laterally fills the space created by a guided chisel used to groove the trochanteric bed.
The improved proximal fit is accomplished in the present invention by having a direct relationship between the prosthesis shape and the shape of the instruments used to prepare the femur. This relationship uses known mathematical techniques to generate surfaces of the proximal prosthesis from the chisel profile and sectional radii of the reamer. This allows the surgeon to prepare the proximal femur knowing that he will have excellent proximal fit, especially posteriorly and laterally.